Power controllers are used in industrial applications to regulate power supplied to machinery, manufacturing equipment, and support systems such as heating and air conditioning systems, for example. In general, a power controller provides the interface between an electrical utility (e.g., user owned power generating facility or other supplier) and the electrical distribution network being served by the power supplied by the electrical utility. The power controller typically functions to supply power to using equipment under normal operating conditions, as well as to interrupt the supply of power in the event of overloads or other extraordinary circumstances, to prevent damage to the network and the equipment connected to the network.
In many industrial applications, power controllers require large, expensive installations involving numerous switches, sensors, and indicators for automatically distributing power through the electrical distribution network. Furthermore, if there is a possible malfunction with the power controller, it is important to be able to shut down the power controller in a controlled manner to prevent damage to the using equipment.
Most large electrical resistance heaters utilize a three-phase circuit in which each circuit is individually fused. For a particular installation, a power controller may handle three-phase voltages of between 208 Vac to 600 Vac, with currents ranging from 50 A to 2500 A. In such applications, a silicon controlled rectifier (SCR) type power controller may be used to regulate the amount of electricity supplied to the resistive load of a heater. For example, the power controller may employ SCRs gated on to allow current flow in a particular direction. Once gate into conduction, each SCR will continue conducting until current flow in the desired direction stops. For alternating current (AC) power distribution networks, each SCR is gated into conduction for each half-cycle of the AC input wave form during which current flows to a particular phase.
The output of a SCR type power controller is often connected to a power distribution unit and to a number of three-phase fuse blocks. For example, the output may be wired to six or eight of such fuse blocks. In addition, the power controller also may include a firing unit or firing package having outputs connected to respective gate inputs of each SCR. In such cases, control or gate inputs for the SCRs are supplied as inputs to the circuitry within the firing packages, which in turn produces output signals for gating the SCRs into conduction at the proper times.
Every component of a power controller consumes valuable panel space and necessitates additional wiring to make electrical connections between components. Because each electrical connection represents a potential problem and increases the cost of a unit by requiring additional labor and materials, minimizing the number of connections is a continuous goal.
To date, power controllers have not been designed so as to minimize connections and reduce initial cost. For example, existing power controllers provide very limited access to components and only few options for running the power wiring. In particular, such power controllers typically include only a single opening in a side of the unit for running wiring in and out of the same end or at most two openings in opposed sides for running wiring in one end and out of the other end. Consequently, it is often necessary to bend power wiring within and/or around the power controller to accommodate various designs. In large power controllers, several square feet of space may be required for bending the power wiring so that proper electrical connections can be made. The space and bending radius of power wiring can easily exceed the footprint of the power controller, resulting in a much larger unit than would otherwise be necessary.
Moreover, in power controllers that are closed or “touch safe,” wiring becomes even more complicated. Power controllers that are “touch safe” generally include a door, hinged top, or other cover for shielding components that become hot to the touch during use. Such protective measures further limit access to components and the ways in which the power wiring can be routed.
Accordingly, there exists the need for a compact, versatile, and “touch safe” power controller having a cost-effective design that accommodates multiple electrical connections for various power applications.
Furthermore, SCR power controllers are not perfect conductors and exhibit some voltage drop across the SCRs. When current is flowing through the SCRs, the voltage drop generates heat. Heat must be removed from the SCRs so that a safe operating temperature is not exceeded causing failure of the controller. Some manufacturers have provided a thermostat mounted on a heat sink to shut down the SCR power controller if temperature approaches the point where the SCR may be damaged. There are two problems with this approach. The first is that thermostats have proved very inaccurate, and the other is that shutting down production with no warning can prove very costly in many industrial processes.
Accordingly, there exists the need for a power controller having an improved mechanical design and self temperature monitoring capabilities.